Notch sensitivity of titanium causing contradictory effects on locked nails and screws

Effects of merged holes, partial thread removal, and offset holes on fatigue strengths of titanium locking plates

BACKGROUND

This study investigated the effects of screw hole merging, thread removal, and screw hole offset on the mechanical properties of locking plates.

METHODS

Finite element models were used to develop the optimal design of the merged holes. Four titanium locking plates with different hole designs were analyzed. Type I had threaded round holes. Type II had merged holes. Type III had merged holes with partial thread removal. Type IV had threaded offset holes. Mechanical experiments similar to finite element analyses were conducted and compared. Screw bending tests were used to assess the screw holding power.

FINDINGS

Finite element analyses showed the optimal merging distance between two round screw holes was 3.5 mm with 2/3 circumferences in each hole. The stresses of types II and III were respectively 6.42% and 7.33%, lower than that of type I. The stress of type IV was 1.66% higher than that of type I. In the mechanical tests, the fatigue lives of types II and III were respectively 3.86 and 7.16 times higher than that of type I. The fatigue life of type IV was 37% lower than that of type I. The differences in the bending strengths of screws were insignificant.

INTERPRETATION

Merging holes could mitigate screw hole stress and increase the fatigue lives of the plates significantly. Partial thread removal could further improve the fatigue life. Merging holes and thread removal did not decrease the screw holding power significantly. The fatigue lives were significantly decreased in plates with offset holes.

Comparison of Early Fatigue Failure of the TFNa and Gamma 3 Cephalomedullary Nails in the United States From 2015 to 2019

OBJECTIVES

To compare reports of implant fatigue failure submitted to the FDA of 2 commonly used cephalomedullary nails.

METHODS

In total, 2724 medical device reports from the FDA's MAUDE database from Jan 2015 to Oct 2019 were reviewed for the Trochanteric Femoral Nail-Advanced (TFNa) and Gamma 3 implants.

RESULTS

Data from 342 implant failures included in the MAUDE database were analyzed. TFNa and Gamma 3 had 183 and 159 reported fatigue failures, respectively. All failed implants fractured in the same location through the proximal screw aperture. Time from implantation to failure was on average 2 months shorter for TFNa implants that were reported fractured than for Gamma 3 implants reported, a difference that was statistically significant (P < 0.05). In total, 100 implants were reported to have failed within the first 4 months (53 and 47 for TFNa and Gamma 3, respectively). For Gamma 3 implants that failed in the first 4 months, almost all of the available manufacturers' inspection reports revealed implant notches at the point of failure from drilling. For TFNa implants that failed early, only one reported notch was noted in the available inspection reports.

CONCLUSIONS

In contrast to other studies regarding fatigue failure, reported failures in both TFNa and Gamma 3 occurred earlier than can be attributed to delayed or nonunion. The reported failures of the TFNa in the MAUDE database occurred earlier than did those of the Gamma 3. Early failures of the Gamma 3 seemed to be the result of iatrogenic implant notching.

LEVEL OF EVIDENCE

Therapeutic Level III. See instructions for authors for a complete description of levels of evidence.

Fatigue Crack Growth and Fracture of Internal Fixation Materials in In Vivo Environments-A Review

The development of crack patterns is a serious problem affecting the durability of orthopedic implants and the prognosis of patients. This issue has gained considerable attention in the medical community in recent years. This literature focuses on the five primary aspects relevant to the evaluation of the surface cracking patterns, i.e., inappropriate use, design flaws, inconsistent elastic modulus, allergic reaction, poor compatibility, and anti-corrosiveness. The hope is that increased understanding will open doors to optimize fabrication for biomedical applications. The latest technological issues and potential capabilities of implants that combine absorbable materials and shape memory alloys are also discussed. This article will act as a roadmap to be employed in the realm of orthopedic. Fatigue crack growth and the challenges associated with materials must be recognized to help make new implant technologies viable for wider clinical adoption. This review presents a summary of recent findings on the fatigue mechanisms and fracture of implant in the initial period after surgery. We propose solutions to common problems. The recognition of essential complications and technical problems related to various approaches and material choices while satisfying clinical requirements is crucial. Additional investigation will be needed to surmount these challenges and reduce the likelihood of fatigue crack growth after implantation.

Half-threaded holes markedly increase the fatigue life of locking plates without compromising screw stability

Aims

To determine whether half-threaded screw holes in a new titanium locking plate design can substantially decrease the notch effects of the threads and increase the plate fatigue life.

Methods

Three types (I to III) of titanium locking plates were fabricated to simulate plates used in the femur, tibia, and forearm. Two copies of each were fabricated using full- and half-threaded screw holes (called A and B, respectively). The mechanical strengths of the plates were evaluated according to the American Society for Testing and Materials (ASTM) F382-14, and the screw stability was assessed by measuring the screw removal torque and bending strength.

Results

The B plates had fatigue lives 11- to 16-times higher than those of the A plates. Before cyclic loading, the screw removal torques were all higher than the insertion torques. However, after cyclic loading, the removal torques were similar to or slightly lower than the insertion torques (0% to 17.3%), although those of the B plates were higher than those of the A plates for all except the type III plates (101%, 109.8%, and 93.8% for types I, II, and III, respectively). The bending strengths of the screws were not significantly different between the A and B plates for any of the types.

Conclusion

Removing half of the threads from the screw holes markedly increased the fatigue life of the locking plates while preserving the tightness of the screw heads and the bending strength of the locking screws. However, future work is necessary to determine the relationship between the notch sensitivity properties and titanium plate design.Cite this article: Bone Joint Res 2020;9(10):645-652.

Modification of the screw hole structures to improve the fatigue strength of locking plates

BACKGROUND

The fatigue fracture of locking plates can substantially threaten fracture treatment results. In the present study, three measures for modifying the screw hole structures of plates were implemented to improve their fatigue strength.

MATERIALS

Custom-made identical titanium locking plates, except the screw hole configurations, were tested using four-point bending load. The three measures were partial removal of screw threads on the tension side of the plates, reduction of screw hole size, and modification of the thread radii. There were six types of plates: control (Type I), half of the threads removed (Type II) or one-third of the threads (Type III), smaller screw holes (Type IV), and increase of the thread root radii (Type V) or crest radii (Type VI).

FINDINGS

Compared with the control, Types II and III significantly improved the fatigue strength (14.5 and 10.1 times, respectively). Decreasing the size of the screw hole (Type IV) also yielded a higher fatigue strength (17.6%). Type VI significantly improved the fatigue strength (9.8 times). However, Type V decreased the fatigue strength (14%). For cyclic stiffness, Type IV was significantly higher than other types statistically. Failure analyses showed typical fatigue fracture in all plates and the cracks were always initiated at the thread crest.

INTERPRETATIONS

The fatigue strength of titanium locking plates can be significantly improved by structural changes in the screw holes. Removing the threads of the plates and increasing the crest radii of the threads were more effective measures than decreasing the size of the screw holes.

Notch sensitivity jeopardizes titanium locking plate fatigue strength

INTRODUCTION

Notch sensitivity may compromise titanium-alloy plate fatigue strength. However, no studies providing head-to-head comparisons of stainless-steel or titanium-alloy locking plates exist.

MATERIALS AND METHODS

Custom-designed identically structured locking plates were made from stainless steel (F138 and F1314) or titanium alloy. Three screw-hole designs were compared: threaded screw-holes with angle edges (type I); threaded screw-holes with chamfered edges (type II); and non-threaded screw-holes with chamfered edges (type III). The plates' bending stiffness, bending strength, and fatigue life, were investigated. The stress concentration at the screw threads was assessed using finite element analyses (FEA).

RESULTS

The titanium plates had higher bending strength than the F1314 and F138 plates (2.95:1.56:1) in static loading tests. For all metals, the type-III plate fatigue life was highest, followed by type-II and type-I. The type-III titanium plates had longer fatigue lives than their F138 counterparts, but the type-I and type-II titanium plates had significantly shorter fatigue lives. All F1314 plate types had longer fatigue lives than the type-III titanium plates. The FEA showed minimal stress difference (0.4%) between types II and III, but the stress for types II and III was lower (11.9% and 12.4%) than that for type I.

CONCLUSIONS

The screw threads did not cause stress concentration in the locking plates in FEA, but may have jeopardized the fatigue strength, especially in the notch-sensitive titanium plates. Improvement to the locking plate design is necessary.

A novel nail providing more biomechanical rotational and axial stability than conventional interlocking nail in femur complex fracture model

PURPOSE

Inter-fragmentary rotational and axial instabilities are major challenges in nailing of complex or comminuted fractures. We aimed to compare the inter-fragmentary rotational and axial stability of novel anti-rotation interlocking nail and the conventional interlocking nail in complex or comminuted femur shaft fractures.

METHODS

Twenty composite femurs were divided into two groups, 30 mm was resected from the mid-portion of all composite femurs. The inter-fragmentary rotational and axial stabilities were assessed.

RESULTS

Between 10-N m external and 6-N m internal rotation torques, mean maximum inter-fragmentary rotational arc motion in the novel nails was 1.63 mm and 291 % less than that of the conventional nails (6.38 mm, P = 0.000). Between 150 N distraction and 2300 N compression, mean axial motion in the novel nails was 0.8 mm and 257 % less than that of the conventional nails (2.86 mm, p = 0.000).

CONCLUSION

An anti-rotational novel nail is superior to the conventional interlocking nail in terms of maximum inter-fragmentary rotational and axial stabilities in complex and comminuted femur shaft fractures.

Fatigue performance of medical Ti6Al4V alloy after mechanical surface treatments

Mechanical surface treatments have a long history in traditional engineering disciplines, such as the automotive or aerospace industries. Today, they are widely applied to metal components to increase the mechanical performance of these. However, their application in the medical field is rather rare. The present study aims to compare the potential of relevant mechanical surface treatments on the high cycle fatigue (R = 0.1 for a maximum of 10 million cycles) performance of a Ti6Al4V standard alloy for orthopedic, spinal, dental and trauma surgical implants: shot peening, deep rolling, ultrasonic shot peening and laser shock peening. Hour-glass shaped Ti6Al4V specimens were treated and analyzed with regard to the material’s microstructure, microhardness, residual stress depth profiles and the mechanical behavior during fatigue testing. All treatments introduced substantial compressive residual stresses and exhibited considerable potential for increasing fatigue performance from 10% to 17.2% after laser shock peening compared to non-treated samples. It is assumed that final mechanical surface treatments may also increase fretting wear resistance in the modular connection of total hip and knee replacements.

Improving stability of locking compression plates through a design modification: a computational investigation

Femoral shaft fractures are common in both the young and elderly due to high-impact trauma and low-impact trauma, respectively. Its treatment by indirect reduction through use of locking compression plates (LCPs) has been on the rise. The LCP possess several advantages in fracture fixation, combining angular stability through use of locking screws with misalignment correction and fracture reduction onto the plate through use of conventional screws. However, there have been cases of plate breakage and fracture non-unions to warrant a study to improve its stability. A design modification is suggested for mid-diaphyseal fractures, whereby unused screw holes are removed. The structural stability of the modified and commercially available LCP is computationally analyzed using finite element modelling and a comparison made in terms of mechanical performance across different fracture lengths. A critical fracture length for which the commercially available LCP is functional as a fixator for mid-diaphyseal fractures was established. The maximum von Mises' stress attained by the commercially available LCP rose to as high as 105 MPa, whereas for the modified LCP, it did not exceed 25 MPa. As expected, these stresses were also found at screw holes, nearest to the fracture site. Critical fracture length allows clinicians to quantitatively distinguish between mid-diaphyseal fractures that can or cannot be treated by the use of LCP fixation. It is also believed that the proposed design modification will substantially increase the fatigue life of the fixator, especially at screw holes nearest to the fracture region, where most fatigue fractures are known to occur and will consequently be functional for greater fracture lengths.